Formation of Sn seeds on indium-free TCO for plating metallization of silicon heterojunction solar cells

The cost-prohibitive ITO and low-temperature silver paste pose significant challenges in the manufacture of SHJ solar cells. The SnO2-based TCO (indium-free) and Cu electroplating are promising solutions to address these issues. The key point of Cu electroplating is the seed layer for achieving good adhesion and low contact resistivity to the TCO. In this study, we report the electrochemical reduction method for the Sn seeds on FTO films, offering an alternative to the costly vacuum deposition process. We reveal that the electrochemical reduction process is dominated by the breaking of lower-energy bonds during electrons injection, resulting in the formation of tin nanoparticles which serve as the excellent seeds for the subsequent electroplating of a nickel metal layer on the FTO film. A tightly binding interface without any pores between the FTO film and Ni was observed, confirming its effectiveness. Our calculation results using the DFT method manifest that the binding energies of Sn/Ni interface are significantly lower than SnO2/Ni, indicating that Sn seeds present a lower energy barrier for deposition, which promotes Ni deposition kinetics. The integration of indium-free TCO, seeds preparation of electrochemical reduction, and copper electroplating is expected to boost SHJ solar cells in near future.